Establishment of postal audit system in intensity-modulated radiotherapy by radiophotoluminescent glass dosimeters and a radiochromic film

We developed an efficient postal audit system to independently assess the delivered dose using radiophotoluminescent glass dosimeters (RPLDs) and the positional differences of fields using EBT3 film at the axial plane for intensity-modulated radiotherapy (IMRT). The audit phantom had a C-shaped target structure as a planning target volume (PTV) with four measurement points for the RPLDs and a cylindrical structure as the organ at risk (OAR) for one measurement point. The phantoms were sent to 24 institutions. Point dose measurements with a 0.6 cm³ PTW farmer chamber were also performed to justify glass dosimetry in IMRT. The measured dose with the RPLDs was compared to the calculated dose in the institution’s treatment planning system (TPS). The mean ± 1.96σ of the ratio of the measured dose with the RPLDs to the farmer chamber was 0.997 ± 0.024 with no significant difference (p = .175). The investigations demonstrated that glass dosimetry was reliable with a high measurement accuracy comparable to the chamber. The mean ± 1.96σ for the dose differences with a reference of the TPS dose for the PTV and the OAR was 0.1 ± 2.5% and −2.1 ± 17.8%, respectively. The mean ± 1.96σ for the right-left and the anterior–posterior direction was −0.9 ± 2.8 and 0.5 ± 1.4 mm, respectively. This study is the first report to justify glass dosimetry for implementation in IMRT audit in Japan. We demonstrate that our postal audit system has high accuracy with a high-level criterion of 3%/3 mm.     

Three-dimensional quality assurance of IMRT prostate plans using gel dosimetry

Intensity modulated radiotherapy (IMRT) is one of the most modern radiation therapy treatment techniques. Although IMRT can deliver high and complex conformational doses to the tumor volume, its implementation requires rigorous quality assurance (QA) procedures that include a dosimetric pre-treatment verification of individual patient planning. This verification usually involves measuring a small volume of absolute dose with an ionization chamber and checking bi-dimensional fluency with an array of detectors. The planning technique has tri-dimensional characteristics, but no tridimensional dosimetry has been established in the clinical routine. One strategy to perform three-dimensional dosimetry is to use polymeric gels associated with magnetic resonance imaging to evaluate dose distribution. Here, we have compared the results of conventional QA procedures involving one- and two-dimensional dosimetry to the results of three-dimensional dosimetry conducted with MAGIC-f gel in 10 cases of prostate cancer IMRT planning. More specifically, we used the gamma index (3%/3 mm) to compare the results of three-dimensional dosimetry to the expected dose distributions obtained with the treatment planning system. Except for one IMRT treatment plan, the gel dosimetry results agreed with the conventional quality control and provided an overview of dose distribution in the target volume.     

Intelligence-guided beam angle optimization in treatment planning of intensity-modulated radiation therapy

An intelligence guided approach based on fuzzy inference system (FIS) was proposed to automate beam angle optimization in treatment planning of intensity-modulated radiation therapy (IMRT). The model of FIS is built on inference rules in describing the relationship between dose quality of IMRT plan and irradiated region of anatomical structure. Dose quality of IMRT plan is quantified by the difference between calculated and constraint doses of the anatomical structures in an IMRT plan. Irradiated region of anatomical structure is characterized by the metric, covered region of interest, which is the region of an anatomical structure under radiation field while beam’s eye-view is conform to target volume. Initially, an IMRT plan is created with a single beam. The dose difference is calculated for the input of FIS and the output of FIS is obtained with processing of fuzzy inference. Later, a set of candidate beams is generated for replacing the current beam. This process continues until no candidate beams is found. Then the next beam is added to the IMRT plan and optimized in the same way as the previous beam. The new beam keeps adding to the IMRT plan until the allowed beam number is reached. Two spinal cases were investigated in this study. The preliminary results show that dose quality of IMRT plans achieved by this approach is better than those achieved by the default approach with equally spaced beam setting. It is effective to find the optimal beam combination of IMRT plan with the intelligence-guided approach.     

Comparative analysis of several detectors for the measurement of radiation transmission and leakage from a multileaf collimator

The multileaf collimator (MLC) is the standard device used to shape radiation beams for 3-d conformal and intensity-modulated radiation therapy (IMRT). Due to the inherent properties of MLC, there is a small amount of radiation transmitted through the leaves, called radiation transmission (RT). Accurate measurements of this radiation are required to commission and validate IMRT-capable treatment planning systems because this radiation may impact the dosimetry of IMRT-calculated dose distributions. This work compares several detectors in the measurement of RT for a micro-multileaf collimation system. The results show that there are statistically significant differences in the measured RT values between detectors from 3.5 to 12.5% for the same MLC model and less than 0.2% relative to the isocentre dose for an open reference field. However, although small in magnitude, these differences may impact the dosimetry of IMRT treatment planning by up to 1.78 Gy to the healthy tissue surrounding the target for a treatment of 60 Gy in 30 fractions. By the later, these differences must be included as a source of uncertainty in IMRT dose delivery. Also, it must be established which detector offers the most reliable results in the measurement of the RT by using Monte Carlo simulation methods.     

Comparison of patient-specific intensity modulated radiation therapy quality assurance for the prostate across multiple institutions

AimTo evaluate the success of a patient-specific intensity modulated radiation therapy (IMRT) quality assurance (QA) practice for prostate cancer patients across multiple institutions using a questionnaire survey.BackgroundThe IMRT QA practice involves different methods of dose distribution verification and analysis at different institutions.Materials and MethodsTwo full-arc volumetric modulated arc therapy (VMAT) plan and 7 fixed-gantry IMRT plan with DMLC were used for patient specific QA across 22 institutions. The same computed tomography image and structure set were used for all plans. Each institution recalculated the dose distribution with fixed monitor units and without any modification. Single-point dose measurement with a cylindrical ionization chamber and dose distribution verification with a multi-detector or radiochromic film were performed, according to the QA process at each institution.ResultsTwenty-two institutions performed the patient-specific IMRT QA verifications. With a single-point dose measurement at the isocenter, the average difference between the calculated and measured doses was 0.5 ± 1.9%. For the comparison of dose distributions, 18 institutions used a two or three-dimensional array detector, while the others used Gafchromic film. In the γ test with dose difference/distance-to-agreement criteria of 3%?3 mm and 2%?2 mm with a 30% dose threshold, the median gamma pass rates were 99.3% (range: 41.7%–100.0%) and 96.4% (range: 29.4%–100.0%), respectively.ConclusionThis survey was an informative trial to understand the verification status of patient-specific IMRT QA measurements for prostate cancer. In most institutions, the point dose measurement and dose distribution differences met the desired criteria.     

In vivo dosimetry of the rectum in image-guided adaptive interstitial-intracavitary brachytherapy of cervix cancer – A feasibility study

Aim and backgroundTo investigate the feasibility of in vivo rectal dosimetry in image-guided adaptive brachytherapy of cervical cancer.Materials and methodsError of measurement of dose rate in a semiconductor diode probe was investigated depending on the distance and angle in water, and on temperature in a polymethyl methacrylate phantom using an Ir-192 source. Furthermore, the difference between the measured and calculated dose was analysed in the interstitial brachytherapy of 30 cervix cancer patients. The relationship between in vivo measured dose, calculated dose in the point of the diode, calculated maximal dose in the point of the diodes and calculated maximal dose of the rectum were examined.ResultsThe dosimeter measured with 85% accuracy at more than 5 cm from the source, but within a closer distance the accuracy decreased significantly. At 45–90° angle, the device measured with a 15% error. The error increased with the temperature, 22% at 35 °C. In 8 cases (26.7%) the maximal dose was measured in the correct diode. The device measured 73% of the calculated dose in the point of the diode. The maximum of the calculated doses of diodes was 60% of the calculated maximal dose. The in vivo measured dose was 35% of the calculated maximal dose.ConclusionsUnder treatment conditions, the semiconductor diode does not provide reliable measured data. The probe pushes the rectal wall closer to the high dose areas and underestimates the dose of it. Semiconductor probe is not recommended for in vivo dosimetry of the rectum in image-guided brachytherapy of cervical cancer.     

Accuracy of dose calculation algorithms for static and rotational IMRT of lung cancer: A phantom study

PurposeTo evaluate the dosimetric accuracy of Pencil beam (PB), Anisotropic Analytical Algorithm (AAA) and Collapsed Cone Convolution Superposition (CCCS) in thoracic tumours for various IMRT techniques.MethodsStep-and-shoot Linac IMRT (IMRT), arc volumetric RapidArc (RA) and Helical Tomotherapy (HT) lung treatments for different clinical situations (mediastinum tumour, single metastasis and multiple metastases) were simulated and calculated with PB/AAA, AAA, CCCS, respectively. Delivery quality assurance plans were first verified in homogeneous media (Cheese phantom and ArcCHECK); then several low-density inhomogeneous phantoms were used: the Multiplug ArcCHECK, the commercial ArcCHECK slightly modified with a low density lung–shape insert and a custom-made slab heterogeneous phantom simulating the thorax region. Absolute doses and planar dose maps were checked to assess the agreement between measured and calculated dose distributions.ResultsIn total, data referred to 195 point dose measurements and 189 planar measurements were considered. Average point absolute deviations <3% were found for all the delivery techniques/dose algorithms. In small targets completely embedded in very low density media, deviations up to 7–10% and 4–5% were found for PB and AAA/CCCS respectively. Excellent results were found for planar measurements in ArcCHECK configurations, where ≥95% of points satisfy the 3%/3 mm acceptance criteria for all the algorithms.ConclusionsA satisfactory agreement (<2%) between planned and measured doses was generally found for CCCS and AAA, excepting the very critical situation of a small tumour completely embedded in air. A significant dose overestimation (from few to 5–7%) was confirmed for PB in complex inhomogeneous arrangements.     

A new cone-beam computed tomography dosimetry method providing optimal measurement positions: A Monte Carlo study

PurposeThe conventional weighted computed tomography dose index (CTDI_w) may not be suitable for cone-beam computed tomography (CBCT) dosimetry because a cross-sectional dose distribution is angularly inhomogeneous owing to partial angle irradiations. This study was conducted to develop a new dose metric (f(0)^CB_w) for CBCT dosimetry to determine a more accurate average dose in the central cross-sectional plane of a cylindrical phantom using Monte Carlo simulations.MethodsFirst, cross-sectional dose distributions of cylindrical polymethyl methacrylate phantoms over a wide range of phantom diameters (8–40 cm) were calculated for various CBCT scan protocols. Then, by obtaining linear least-squares fits of the full datasets of the cross-sectional dose distributions, the optimal radial positions, which represented measurement positions for the average phantom dose, were determined. Finally, the f(0)^CB_w method was developed by averaging point doses at the optimal radial positions of the phantoms. To demonstrate its validity, the relative differences between the average doses and each dose index value were estimated for the devised f(0)^CB_w, conventional CTDI_w, and Haba’s CTDI_w methods, respectively.ResultsThe relative differences between the average doses and each dose index value were within 4.1%, 16.7%, and 11.9% for the devised, conventional CTDI_w, and Haba’s CTDI_w methods, respectively.ConclusionsThe devised f(0)^CB_w value was calculated by averaging four “point doses” at 90° intervals and the optimal radial positions of the cylindrical phantom. The devised method can estimate the average dose more accurately than the previously developed CTDI_w methods for CBCT dosimetry.     

Comparison of planned and measured rectal dose in-vivo during high dose rate Cobalt-60 brachytherapy of cervical cancer

Cobalt-60 (Co-60) is a relatively new source for the application of high-dose rate (HDR) brachytherapy. Radiation dose to the rectum is often a limiting factor in achieving the full prescribed dose to the target during brachytherapy of cervical cancer. The aim of this study was to measure radiation doses to the rectum in-vivo during HDR Co-60 brachytherapy. A total of eleven HDR brachytherapy treatments of cervical cancer were recruited in this study. A series of diodes incorporated in a rectal probe was inserted into the patient's rectum during each brachytherapy procedure. Real-time measured rectal doses were compared to calculated doses by the treatment planning system (TPS). The differences between calculated and measured dose ranged from 8.5% to 41.2%. This corresponds to absolute dose differences ranging from 0.3 Gy to 1.5 Gy. A linear relationship was observed between calculated and measured doses with linear regression R² value of 0.88, indicating close association between the measured and calculated doses. In general, absorbed doses for the rectum as calculated by TPS were observed to be higher than the doses measured using the diode probe. In-vivo dosimetry is an important quality assurance method for HDR brachytherapy of cervical cancer. It provides information that can contribute to the reduction of errors and discrepancies in dose delivery. Our study has shown that in-vivo dosimetry is feasible and can be performed to estimate the dose to the rectum during HDR brachytherapy using Co-60.     

Multi-institutional evaluation using the end-to-end test for implementation of dynamic techniques of radiation therapy in Thailand

AimIn this study, an accuracy survey of intensity-modulated radiation therapy (IMRT) and volumetric arc radiation therapy (VMAT) implementation in radiotherapy centers in Thailand was conducted.BackgroundIt is well recognized that there is a need for radiotherapy centers to evaluate the accuracy levels of their current practices, and use the related information to identify opportunities for future development.Materials and methodsAn end-to-end test using a CIRS thorax phantom was carried out at 8 participating centers. Based on each center's protocol for simulation and planning, linac-based IMRT or VMAT plans were generated following the IAEA (CRP E24017) guidelines. Point doses in the region of PTVs and OARs were obtained from 5 ionization chamber readings and the dose distribution from the radiochromic films. The global gamma indices of the measurement doses and the treatment planning system calculation doses were compared.ResultsThe large majority of the RT centers (6/8) fulfilled the dosimetric goals, with the measured and calculated doses at the specification points agreeing within ±3% for PTV and ±5% for OARS. At 2 centers, TPS underestimated the lung doses by about 6% and spinal cord doses by 8%. The mean percentage gamma pass rates for the 8 centers were 98.29 ± 0.67% (for the 3%/3 mm criterion) and 96.72 ± 0.84% (for the 2%/2 mm criterion).ConclusionsThe 8 participating RT centers achieved a satisfactory quality level of IMRT/VMAT clinical implementation.     

Energy dependence of radiochromic dosimetry films for use in radiotherapy verification

Aim

The purpose of the study was to examine the energy dependence of Gafchromic EBT radiochromic dosimetry films, in order to assess their potential use in intensity-modulated radiotherapy (IMRT) verifications.

Materials and methods

The film samples were irradiated with doses from 0.1 to 12 Gy using photon beams from the energy range 1.25 MeV to 25 MV and the film response was measured using a flat-bed scanner. The samples were scanned and the film responses for different beam energies were compared.

Results

A high uncertainty in readout of the film response was observed for samples irradiated with doses lower than 1 Gy. The relative difference exceeds 20% for doses lower than 1 Gy while for doses over 1 Gy the measured film response differs by less than 5% for the whole examined energy range. The achieved uncertainty of the experimental procedure does not reveal any energy dependence of Gafchromic EBT film response in the investigated energy range.

Conclusions

Gafchromic EBT film does not show any energy dependence in the conditions typical for IMRT but the doses measured for pre-treatment plan verifications should exceed 1 Gy.     

The influence of respiratory motion on dose distribution in accelerated partial breast irradiation using volumetric modulated arc therapy

PurposeAccelerated partial breast irradiation (APBI) is alternative treatment option for patients with early stage breast cancer. The interplay effect on volumetric modulated arc therapy APBI (VMAT-APBI) has not been clarified. This study aimed to evaluate the feasibility of VMAT-APBI for patients with small breasts and investigate the amplitude of respiratory motion during VMAT-APBI delivery that significantly affects dose distribution.MethodsThe VMAT-APBI plans were generated with 28.5 Gy in five fractions. We performed patient-specific quality assurance using Delta⁴ phantom under static conditions. We also measured point dose and dose distribution using the ionization chamber and radiochromic film under static and moving conditions of 2, 3 and 5 mm. We compared the measured and calculated point doses and dose distributions by dose difference and gamma passing rates.ResultsA total of 20 plans were generated; the dose distributions were consistent with those of previous reports. For all measurements under static conditions, the measured and calculated point doses and dose distributions showed good agreement. The dose differences for chamber measurement were within 3%, regardless of moving conditions. The mean gamma passing rates with 3%/2 mm criteria in the film measurement under static conditions and with 2 mm, 3 mm, and 5 mm of amplitude were 95.0 ± 2.0%, 93.3 ± 3.3%, 92.1 ± 6.2% and 84.8 ± 7.8%, respectively. The difference between 5 mm amplitude and other conditions was statistically significant.ConclusionsRespiratory management should be considered for the risk of unintended dose distribution if the respiratory amplitude is >5 mm.     

GafChromic® EBT3 films for patient specific IMRT QA using a multichannel approach

PurposeTo evaluate EBT3 for pre-treatment patient specific quality assurance (QA). The method we propose combines the experience gained in our center with the guidelines of the protocol proposed by Lewis et al. in 2012. To compare the multichannel approach with the single channel dosimetry.MethodsGafchromic® EBT3 films were irradiated both at linac and TomoTherapy and calibration curves were obtained. A series of irradiations with simple fields (uniform dose distributions on regular shaped targets) was performed. In a second stage, films were exposed to full clinical plans at linac (step and shoot IMRT and VMAT). At TomoTherapy dose maps were obtained for a clinical plan in three different coronal planes. Films were digitized using an Epson 10000XL scanner and FilmQA™ Pro software was employed for the analysis.ResultsThe measured calibration curves suggest that, at least for the two beams taken into account (6 MV linac and TomoTherapy), a single calibration can be successfully adopted for each film lot. The application of the multichannel optimization method strongly improves the results in terms of gamma passing rates of the comparison between measured and calculated maps.ConclusionsUp to now EBT films, although attractive, were not preferred for routine patient specific QA due to their complex and time consuming processing and to the challenging work of characterization. The application of the mentioned protocol, together with some additional precautions, and the adoption of the multichannel optimization dosimetry, make this detector a handy and reliable tool for patient specific QA.     

Brain-Sparing Methods for IMRT of Head and Neck Cancer

Purpose

Radical radiotherapy for head and neck cancer (HNC) may deliver significant doses to brain structures. There is evidence that this may cause a decline in neurocognitive function (NCF). Radiation dose to the medial temporal lobes, and particularly to the hippocampi, seems to be critical in determining NCF outcomes. We evaluated the feasibility of two alternative intensity-modulated radiotherapy (IMRT) techniques to generate hippocampus- and brain-sparing HNC treatment plans to preserve NCF.

Methods and Materials

A planning study was undertaken for ten patients with HNC whose planning target volume (PTV) included the nasopharynx. Patients had been previously treated using standard (chemo)-IMRT techniques. Bilateral hippocampi were delineated according to the RTOG atlas, on T1w MRI co-registered to the RT planning CT. Hippocampus-sparing plans (HSRT), and whole-brain/hippocampus-sparing fixed-field non-coplanar IMRT (BSRT) plans, were generated. DVHs and dose difference maps were used to compare plans. NTCP calculations for NCF impairment, based on hippocampal dosimetry, were performed for all plans.

Results

Significant reductions in hippocampal doses relative to standard plans were achieved in eight of ten cases for both HSRT and BSRT. EQD2 D40% to bilateral hippocampi was significantly reduced from a mean of 23.5 Gy (range 14.5–35.0) in the standard plans to a mean of 8.6 Gy (4.2–24.7) for HSRT (p = 0.001) and a mean of 9.0 Gy (4.3–17.3) for BSRT (p < 0.001). Both HSRT and BSRT resulted in a significant reduction in doses to the whole brain, brain stem, and cerebellum.

Conclusion

We demonstrate that IMRT plans for HNC involving the nasopharynx can be successfully optimised to significantly reduce dose to the bilateral hippocampi and whole brain. The magnitude of the achievable dose reductions results in significant reductions in the probability of radiation-induced NCF decline. These results could readily be translated into a future clinical trial.     

A novel method for dose distribution registration using fiducial marks made by a megavoltage beam in film dosimetry for intensity-modulated radiation therapy quality assurance

PurposePhotographic film is widely used for the dose distribution verification of intensity-modulated radiation therapy (IMRT). However, analysis for verification of the results is subjective. We present a novel method for marking the isocenter using irradiation from a megavoltage (MV) beam transmitted through slits in a multi-leaf collimator (MLC).MethodsWe evaluated the effect of the marking irradiation at 500 monitor units (MU) on the total transmission through the MLC using an ionization chamber and Radiochromic Film. Film dosimetry was performed for quality assurance (QA) of IMRT plans. Three methods of registration were used for each film: marking by irradiating with an MV beam through slits in the MLC (MLC-IC); marking with a fabricated phantom (Phantom-IC); and a subjective method based on isodose lines (Manual). Each method was subjected to local γ-analysis.ResultsThe effect of the marking irradiation on the total transmission was 0.16%, as measured by a ionization chamber at a 10-cm depth in a solid phantom, while the inter-leaf transmission was 0.3%, determined from the film. The mean pass rates for each registration method agreed within ±1% when the criteria used were a distance-to-agreement (DTA) of 3 mm and a dose difference (DD) of 3%. For DTA/DD criteria of 2 mm/3%, the pass rates in the sagittal plane were 96.09 ± 0.631% (MLC-IC), 96.27 ± 0.399% (Phantom-IC), and 95.62 ± 0.988% (Manual).ConclusionThe present method is a versatile and useful method of improving the objectivity of film dosimetry for IMRT QA.     

Detailed evaluation of Mobius3D dose calculation accuracy for volumetric-modulated arc therapy

PurposeTo perform a detailed evaluation of dose calculation accuracy and clinical feasibility of Mobius3D. Of particular importance, multileaf collimator (MLC) modeling accuracy in the Mobius3D dose calculation algorithm was investigated.MethodsMobius3D was fully commissioned by following the vendor-suggested procedures, including dosimetric leaf gap (DLG) optimization. The DLG optimization determined an optimal DLG correction factor which minimized the average difference between calculated and measured doses for 13 patient volumetric-modulated arc therapy (VMAT) plans. Two sets of step-and-shoot plans were created to examine MLC and off-axis open fields modeling accuracy of the Mobius3D dose calculation algorithm: MLC test set and off-axis open field test set. The test plans were delivered to MapCHECK for the MLC tests and an ionization chamber for the off-axis open field test, and these measured doses were compared to Mobius3D-calculated doses.ResultsThe mean difference between the calculated and measured doses across the 13 VMAT plans was 0.6% with an optimal DLG correction factor of 1.0. The mean percentage of pixels passing gamma from a 3%/1 mm gamma analysis for the MLC test set was 43.5% across the MLC tests. For the off-axis open field tests, the Mobius3D-calculated dose for 1.5 cm square field was −4.6% lower than the chamber-measured dose.ConclusionsIt was demonstrated that Mobius3D has dose calculation uncertainties for small fields and MLC tongue-and-groove design is not adequately taken into consideration in Mobius3D. Careful consideration of DLG correction factor, which affects the resulting dose distributions, is required when commissioning Mobius3D for patient-specific QA.     

Thermoluminescence dosimetry of the dose received by scrotum and testes in radiotherapy of rectal cancer,compared to the point doses calculated by 3D-planning software

PurposeRadiation received by the testes in the course of radiotherapy for rectal cancer may cause oligospermia and azospermia. We sought to determine the dose to the scrotum and testes with thermoluminescence dosimetry (TLD), and compare it to the dose calculated by 3D planning software.MethodsThe TLDs were fixed to the scrotum in six points anteriorly and posteriorly in two fractions of radiotherapy. All patients received a 50–50.4 Gy total dose in prone position with 3D-planning. The average dose of TLD measurements was compared to the average of 6 relevant point doses calculated by the planning software.ResultsThe mean scrotal dose of radiation in 33 patients as measured by TLD was 3.77 Gy (7.5% of the total prescribed dose), and the mean of point doses calculated by the planning software was 4.11 Gy (8.1% of the total dose), with no significant difference. A significant relationship was seen between the position of the inferior edge of the fields and the mean scrotal dose (P = .04). Also body mass index (BMI) was inversely related with the scrotal dose (P = .049).ConclusionWe found a dose of about 4 Gy received by the scrotum and testes from a total prescribed dose of 50 Gy in the radiotherapy of rectal carcinoma patients, with TLD measurements confirming testicular dose estimations by the planning software. This dose could be significantly harmful for spermatogenesis. Thus careful attention to the testicular dose in radiotherapy of rectal cancer for men desiring continued fertility is a necessity.     

Analyzing the performance of ArcCHECK diode array detector for VMAT plan

Aim

The aim of this study is to evaluate performance of ArcCHECK diode array detector for the volumetric modulated arc therapy (VMAT) patient specific quality assurance (QA). VMAT patient specific QA results were correlated with ion chamber measurement. Dose response of the ArcCHECK detector was studied.

Background

VMAT delivery technique improves the dose distribution. It is complex in nature and requires proper QA before its clinical implementation. ArcCHECK is a novel three dimensional dosimetry system.

Materials and methods

Twelve retrospective VMAT plans were calculated on ArcCHECK phantom. Point dose and dose map were measured simultaneously with ion chamber (IC-15) and ArcCHECK diode array detector, respectively. These measurements were compared with their respective TPS calculated values.

Results

The ion chamber measurements are in good agreement with TPS calculated doses. Mean difference between them is 0.50% with standard deviation of 0.51%. Concordance correlation coefficient (CCC) obtained for ion chamber measurements is 0.9996. These results demonstrate a strong correlation between the absolute dose predicted by our TPS and the measured dose. The CCC between ArcCHECK doses and TPS predictions on the CAX was found to be 0.9978. In gamma analysis of dose map, the mean passing rate was 98.53% for 3% dose difference and 3 mm distance to agreement.

Conclusions

The VMAT patient specific QA with an ion chamber and ArcCHECK phantom are consistent with the TPS calculated dose. Statistically good agreement was observed between ArcCHECK measured and TPS calculated. Hence, it can be used for routine VMAT QA.     

Comparison of the extent of hippocampal sparing according to the tilt of a patient’s head during WBRT using linear accelerator-based IMRT and VMAT

In this paper, we report the results of our investigation into whole brain radiotherapy (WBRT) using linear accelerator-based intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) in lung cancer patients with a high risk of metastasis to the brain. Specifically, we assessed the absorbed dose and the rate of adverse effects for several organs at risk (OAR), including the hippocampus, according to the tilt of a patient’s head. We arbitrarily selected five cases where measurements were made with the patients’ heads tilted forward and five cases without such tilt. We set the entire brain as the planning target volume (PTV), and the hippocampi, the lenses, the eyes, and the cochleae as the main OAR, and formulated new plans for IMRT (coplanar, non-coplanar) and VMAT (coplanar, non-coplanar). Using the dose-volume histogram (DVH), we calculated and compared the effective uniform dose (EUD), normal tissue complication probability (NTCP) of the OAR and the mean and the maximum doses of hippocampus. As a result, if the patient tilted the head forward when receiving the Linac-based treatment, for the same treatment effect in the PTV, we confirmed that a lower dose entered the OAR, such as the hippocampus, eye, lens, and cochlea. Moreover, the damage to the hippocampus was expected to be the least when receiving coplanar VMAT with the head tilted forward. Accordingly, if patients tilt their heads forward when undergoing Linac-based WBRT, we anticipate that a smaller dose would be transmitted to the OAR, resulting in better quality of life following treatment.